The present invention concerns a method for adjusting the oscillation frequency of a sprung balance fitted to a mechanical timepiece. The regulating assembly of a clockwork movement will be described briefly hereinafter with reference to annexed FIGS. 1 and 2. In a conventional manner, this regulating assembly includes a balance spring 1 associated with a balance 2. The balance spring is connected to the balance staff 4 by means of a collet 6. The outer end 8 of the balance spring is fixed to a balance-spring stud 10 secured to the balance cock 12. The oscillation of the sprung balance is maintained by the escapement wheel 14 meshed with the gear train of the movement.
In order to adjust the oscillation frequency of the sprung balance, two pins 16 and 17 are conventionally provided, fixed to an index or regulator 18 capable of being moved in rotation for example by means of an eccentric screw 20. Thus, by varying the position of the pins, the active length of the balance spring is varied. As the elastic torque of the balance spring is inversely proportional to the active length of the balance spring, the oscillation frequency of the sprung balance is substantially inversely proportional to the square root of this active length. Thus, in order to adjust the working frequency of the clockwork movement, a adjusting system comprising an index and pins between which the end portion 22 of the outer turn 24 of said balance spring 1, is generally provided in the prior art.
It will be noted that other adjusting systems exist, which act in particular on the moment of inertia of balance 2. All these means require the arrangement of a certain number of components intended for adjusting the oscillation frequency of the sprung balance.
Moreover, in the case shown in FIGS. 1 and 2, the adjustment of the two pins 16 and 17 is relatively difficult. It is often noted that depending on the position of the movement in space, the action of the pins is more or less efficient so that the active length of the balance spring varies according to said position. This is of course detrimental to the working of the timepiece and thus for the precision of the time or time interval displayed by the latter.
The object of the present invention consists in overcoming the aforementioned drawbacks by proposing an efficient, inexpensive method for adjusting the frequency of a sprung balance which allows the construction of the regulating assembly to be simplified.
The invention thus concerns a method for adjusting the oscillation frequency of a sprung balance in accordance with the steps mentioned in claim 1, during which laser machining of the balance spring is used to reduce the elastic torque of the latter until a reference elastic torque substantially corresponding to the reference frequency for the oscillation of the sprung balance is reached. It will be noted here that knowledge of the reference elastic torque is not necessary given that the adjustment can be performed in one or more successive steps by measuring the frequency of the assembled sprung balance. However, within the scope of the present invention, when ready to be assembled, the balance spring must have, for a given length between the fixing points to collet 6 and stud 10, a greater elastic torque than the aforementioned reference elastic torque. Indeed, the effect of the laser machining is to reduce the elastic torque of the spring, as will be explained hereinafter in more detail.
It will be noted that the oscillation frequency of the sprung balance may be adjusted during a preliminary step, preceding the assembly of the sprung balance in a clockwork movement. In a variant, pre-adjustment may be performed during this preliminary step and fine adjustment, also using a laser beam, is performed once the sprung balance is mounted in the clockwork movement to allow the oscillation frequency of the sprung balance to be precisely adjusted in a real operating situation and thus precise working of the clockwork movement to be assured.
As a result of the features of the present invention, no specific element for adjusting the oscillation frequency of the sprung balance is necessary. In particular, the regulating assembly preferably no longer includes any index pins. Thus, working variation problems depending on the position of the movement, due to the passing of the outer turn of the balance spring between the pins, are removed. It will be noted however that it is possible, in a particular embodiment, to fit the balance with fine adjusting means of its moment of inertia so as to allow subsequent adjustment after a certain period of operation of the clockwork movement. Indeed, the present invention is particularly suited to adjustment in a watchmaking factory or work shop and may prove complex for implementation by an after-sales service shop which performs conventional checks and necessary repairs following any damage caused to the timepiece.